how to fill this gap?
If I understand right you ask about the right topology. If so it's not good that you have inconsistent mesh (by one axis you have far more loop cuts compare to another). If there is a reason for that I would probably fill it like that:

The big N-gon in the middle is fine if this area is flat.
But the better solution would be to have more consistent mesh so you can connect loops organically. You'll still have 1 triangle but that is fine.

I hope it helps!